GermanMonte Posted March 13, 2015 Share Posted March 13, 2015 Hello from Germany, over the past few days I was trying to adjust my carburetor a little bit. Background: I bought my motor as a turnkey crate engine from TriStar about a year ago. I usually like to build my engines myself, but this was the best option for me sitting here in Germany, since I did not want to deal with shipping part after part to Germany to build one myself and having to spend a freaking fortune on taxes and shipping costs. Anyway, I've been running it with two AFR gauges for each bank and realized, that it was running pig rich at almost every rpm and any load. I decided to get this right and took a closer look at the setup of the carb (QuickFuel SS-750 Super Street with mechanical secondaries). Motor: 427 SBC Dart Block Vacuum at idle: Erratic, but between 6 and 7 inHG Cam: Howards Intake Valve Lift: .545 Duration Intake: 294 Exhaust Valve Lift: .565 Duration Exhaust: 300 Lobe Center: 110 Intake Center Line: 106 Carb: Primary Main Jet: Used to be 72, now 70 Secondary Jet: Uset to be 75, now 74 Idle Air Bleed: 70/70 High Speed Bleed: 31/31 Primary Nozzle: 31 Secondary Nozzle: 31 Power Valve: Used to be 6.5, now 3.5 Four corner idle adjusment Secondaries open at 60% of the primaries right now Before I started messing with it, idle AFR was between 10 and 11. Part throttle, low rpm AFR was around 12.5 and WOT at slightly over 11. That's how it came from Tristar (no complaints here, it's a well built engine with high quality components - it's just the carb that's not adjusted properly). First thing I did was leaning out the idle a little bit and putting a vacuum-correct Power Valve in the carb, so that it doesn't add fuel while idling. Now it idles at AFR 13.5 (all four idle screws adjusted with the help of the vaccum gauge and the sensors). Part throttle is close to AFR 14 and WOT is still a bit rich at 11.5. WOT is not my concern right now, since I can raise the AFR reading easily with the secondary jets. What bothers me now is that it completely leans out at light throttle and low rpm (cruising at low speeds with the converter clutch shut or accelerating slightly). At that particular point I have AFR readings between 16 and 18 and the vacuum gauge shows a flat 10 inHg (power valve not open naturally). It starts to stumble and wants do die on me. It goes away as soon as I push down the throttle a bit further though and apply some load (I guess when the primary jets kick in). Swapping power valves back and forth of course didn't cure it. I have to richen up the idle screws to the original AFR reading to get rid of the lean condition. I don't wanna have to decide between a pig rich idle and a lean condition. Swapping the primaries for bigger ones would pull down AFR at part throttle again.There's gotta be a solution! Would it help to swap the idle air bleeds for smaller ones? What are your ideas on this problem? I've got to mention that I didn't measure the transfer slot exposure yet, but if my memory serves me correct then they looked square on both the primaries and the secondaries. Hope you can guide me through this. Thanks a lot! Robert Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted March 13, 2015 Author Share Posted March 13, 2015 Btw: Idle mixture screws are out roughly one turn at the moment. They're not adjusted evenly regarding turns, but evenly regarding mixture and vacuum. Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted March 13, 2015 Share Posted March 13, 2015 what's the cam duration @ .050? and the compression ratio? I've had the same issue with that same carb there are a few things here you can try first I would try smaller idle air bleeds to richen up the off idle, then you'll have to close up the idle jets some here's some very good information http://ls1tech.com/forums/carbureted-lsx-forum/1626345-next-level-holley-tuning-serious-ocd-tuners.html Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted March 13, 2015 Author Share Posted March 13, 2015 Conpression is 10:1. Intake duration at cam at 0.050 lift is 241 degrees, exhaust is 247 degrees. From my understanding I need to get the transition circuit right, but there's little information out there how to do it. Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted March 13, 2015 Share Posted March 13, 2015 follow the link I posted, and the links in it Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted March 14, 2015 Author Share Posted March 14, 2015 Will do! I pulled the carb off of the motor today and checked the transfer slots for adjustment. The first stage showed a lot of the slot, on the second stage they were completely covered by the throttle blades. Both sides are adjusted to the suggested settings. Might have been the reason for the cars behavior. I didn't have the chance to start the car today though, it was just too cold in my shop and my heater quit working. I'll let you know when there's progress. Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted March 17, 2015 Author Share Posted March 17, 2015 News from the shop: Adjusting the throttle blades to spec (transfer slot barely exposed, square looking) didn't help. I read through the posts you listed here and found out, that I've gotta put my attention on the transfer slot mixture. Like I said, AFR at idle is close to 14, part throttle now is as well, but off idle (driving on the transfer slot) is way too lean. I will try out some bigger idle feed restrictors first and adjust the idle air feeds accordingly. I'll keep you guys posted! Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted March 17, 2015 Share Posted March 17, 2015 how far (turns) out are your idle mixture screws? going a little richer on your idle might be necessary, your cam would probably appreciate it with some engine combinations you can't go by "the standard rules" moving the idle restrictor to the lower spot as they suggested might help too, and did you read about the emulsion ports too? Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted March 17, 2015 Author Share Posted March 17, 2015 Idle mixture screws are out about 1/2 turn. I know I will have to richen it up a bit with that cam, but I had to quit for the day. You can tell that the motor runs smoother with some extra fuel at idle. I've read through the articles, but I had to consult my dictionary quite a bit. It's not easy being a German . I've got to order up some air bleeds, idle air restrictors and emulsion port plugs. Unfortunately, they don't sell'em over here - thank god there's Summit. Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted April 9, 2015 Author Share Posted April 9, 2015 A little update from the carb surgery: First off, I didn't plug any of the emulsion ports nor did I move the IFR below float level - I thought I might be able to accomplish it by messing with the removable bleeds first. My English is not the worst in the world, but I had a hard time understanding the influence of the emulsion ports. Long story short: No matter what I did, I wasn't able to get it to run properly at low rpm and little load. The problem child is the spot right around 1000 rpm with the converter clutch shut and a little touch to the pedal (vacuum is 5 in.Hg here btw.). It leans out big time (AFR around 16 to 18) and wants die on me. The problem goes away when I reach higher engine speeds or push down the pedal far enough until the primary mains kick in / the power valve opens. The only way to avoid this condition is to fatten up idle to an eye burning level - it didin't matter which combination of IFRs and IABs had been installed. Here's what I did: - First step was using a smaller IFR (from 0.031 to 0.028) without touching the IAB (0.070). This combination naturally leaned out the emulsion mixture, so I had to back out the idle screws a bit to richen it up again (now they were out roughly one turn). The problem remained: Idle was good, transition from idle to high idle was good and transition at higher rpms with a little load was good as well. That stupid spot at 1000 rpm with some load remained the same though. - Next step, out of curiousity was to go even smaller on the IFR. I stepped down to 0.025 and kept the stock IAB 0.070. As expected, it leaned out idle even more and the transition mixture started to get too lean, even without load. Going down on the IAB helped a bit, but there was too little fuel available for it to run properly on the transition slot, even with the screws backed out a little more. - Third step: Going bigger on the IFR (0.033) with the 0.070 IAB. Richer idle, richer transition. It required turning in the idle screws a bit, but the mentioned problem remained. Going bitter on the IAB (0.074) leaned out the emulsion again, but didn't help. - Fourth step, 0.035 IFR with 0.076 IAB: Same [censored], different combination. - Fifth step: 0.037 IFS combined with 0.076 IAB and later with a 0.080 IAB. It richened up the emulsion in the metering plate so much, that even the slightest turn on the idle screws moved the AFR readings up and down big time. I got it to idle as desired though but the problem remained. In fact it's just that single spot / range right around 1000 rpm with some load where it keeps resisting to run properly. To be honest: I'm out of ideas now. I can't believe that I have to pick a choice between an overly rich idle with a good behavior at that stupid spot or a great idle with a stupid transition. It almost looks like if it needs that extra fuel from the idle screws to survive in low rpm conditions with a little load on it. It should pull its mixture from the transition slot instead of pulling it all from the screws. I know that it's a pretty radical cam which doesn't create the best low rpm behavior, but it should be possible to get it to run acceptable. Any ideas? Robert Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted April 9, 2015 Author Share Posted April 9, 2015 Something just dawned on me: I've opened up the secondaries and closed the primaries when I was making sure the throttle blades are showing the correct amount of the transfer slots. By opening the secondaries, don't you reduce the already weak signal on the primaries even more? I mean idle vaccum is a ridiculous 6 to 7 in.Hg and I guess the primary blades / jets / slots could use all the signal they can get, right? It might be worth to shut down the secondaries a little bit again and see what happens on the primaries - damn, will this motor ever get outta my head :-) Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted April 9, 2015 Share Posted April 9, 2015 your low idle vacuum is exactly why you need to relocate the idle restricter to the lower position as those articles I posted said I will be do that to mine when I get to it Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted April 14, 2015 Author Share Posted April 14, 2015 Oh, ok!. I just ordered up a set of 6-32 taps to be able to relocate the IFRs to the lower position. I'll let you know when there's progress. Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted April 14, 2015 Author Share Posted April 14, 2015 Update: I sent the IFRs diving today. The big difference are more steady AFR readings and it reacts better to modifications on the air bleeds. Here's where I'm now: - 0.030 IFRs, submerged on both the primary and the secondary side - 0.068 IABs - Idle screws are out one turn each - No other modifications AFR readings: - Idle in gear: 13 flat - Slightly off idle (with the car standing still): 12 - Cruise with light load at higher rpms: slightly above 14 - Cruise with a little load at around 1000 rpm: 17 and stuttering!!! Like I said, I can now lean out or richen up the idle / transition circuit with the air bleeds (didn't mess with the size of the IFRs today) and the carburetor answers much more noticeably than before, but that pesky spot is still there. The stuttering goes away when I open up the blades a good bit while driving (standing on the brake to avoid raising engine speed), vacuum drops below 3 and that's exactly when the power valve starts to open. Any ideas? Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted April 15, 2015 Share Posted April 15, 2015 probably in the emulsion bleeds, sounds like the main metering needs to start a little earlier Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted April 15, 2015 Author Share Posted April 15, 2015 I'm running out of ideas - and of fuel again. I went through two tanks of gas already. I'm wondering why none of the six emulsion ports in the primary metering plate has plugs or restrictions in it at the moment and I wish I would understand the effect of restricting / plugging them better. Quote Link to comment Share on other sites More sharing options...
GermanMonte Posted May 27, 2015 Author Share Posted May 27, 2015 News from the Kraut factory: I finally got it figured out. The motor runs like a champ now, AFR readings are around 14 at idle and while cruising. Soft acceleration is slightly below 14 and WOT is a perfect 12.5. I definitely learned a lot about carburetors this way. My biggest problem was in the transition circuit. The big 70 IAB delayed it too much, causing that lean condition at low rpm. A smaller idle fuel restriction combined with a much smaller idle air bleed made the motor much more responsive at cruise conditions. I will post up my current setup soon, since I'm still optimizing the start of the main circuit a little bit. Thanks for your help! Robert Quote Link to comment Share on other sites More sharing options...
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